1. Field of the Invention
This invention relates generally to semiconductor devices, and more specifically, to a solderable contact for silicon carbide semiconductor devices.
2. Description of the Prior Art
Silicon carbide (SiC) is becoming a mature technology for high power devices and is used to fabricate advanced devices such as Schottky diodes, transistor JFETs, and MOSFETs, for example. Notably, SiC devices can be packaged such that wire bonds, for example, connect the electrodes of the device to a device package leadframe. However, to fully exploit the high performance characteristics of a SiC device, device package types are required such that one or more electrodes of the device are electrically and mechanically connected directly to the device package leadframe, for example, or are connected through clips/straps to the device package leadframe, for example. Device packages of these types may include standard solderable packages, flipped SiC packages, clip attach packages, and the DirectFET® package.
Notably, in order to form these types of direct connections to the electrodes of a SiC device, a conductive adhesive, such as solder or the like, is required. However, one or more electrodes of a SiC device are often formed of a metal, such as aluminum, that does not readily adhere to solder. As such, in order to form a solder-based connection to these electrodes, a solderable contact is often formed directly on the surface of the electrode and the package connection, for example, then made to this solderable contact. As an example, the solderable contact may be an alloy that contains silver.
As is known, the electrodes of a device may be insulated from other surfaces of the device, such as device terminations, by an insulation passivation layer. Notably, forming a reliable passivation layer in conjunction with a solderable contact as just described may be difficult. For example, the metal deposition, cleaning, and etching steps required to form the solderable contact may damage or alter the passivation/termination layer.
In addition, it has been found that under prolonged exposure to electric fields and moisture, silver ions, for example, from a solderable contact can migrate and form dendrites. This migration is referred to as metal electromigration. Notably, when solder is applied to the surfaces of a solderable contact in order to attach an electrode to a device package, for example, the solder will typically dissolve the exposed silver along the surfaces of the contact, forming a solder alloy. As a result, the silver becomes captured within the alloy and cannot migrate from the solderable contact to create dendrites.
However, the passivation layer of a device often overlaps, for example, the edges of the electrodes. As a result, the passivation layer may abut/contact the solderable contact on a given electrode and conceal portions of the outer surfaces of the contact, preventing the silver along these surfaces from being reached during soldering. This silver may be a source of migrating ions, which may migrate over the passivation layer and form dendrites. Over time, these dendrites may damage the passivation layer, reducing device reliability. As an example, the dendrites may form conductive bridges between the device electrodes and the device terminations.
Accordingly, it would be desirable to provide a solderable contact that does not affect the reliability of a SiC device.